1. EFFECT OF CARBON CONTENT ON GRAIN REFINEMENT OF C-Mn AND IF
STEELS DEFORMED AT WARM WORKING TEMPERATURE
Silva Neto, O. V. and Balancin, O.
Departamento de Engenharia de Materiais – Universidade Federal de São Carlos – DEMa/UFSCar
Rodovia Washington Luiz, Cx.P. 676, Km 235, 13385-000 – São Carlos-SP-Brasil.
Thermomechanical processing has been applied on low carbon steels to produce ultrafine ferrite grains,
which increase both strength and toughness [1,2]. In this report, heavy strains were applied on four kinds of
steels bellow their finish transformation temperatures, γ→α (AR1 ), and the effect of deformation on ferrite
grain refinement was investigated. Although each one material has different AR1 temperature due to the
difference on carbon content, in every cases the materials were strained on ferritic field.
When steels with ferritic structure are strained at intermediary temperatures, the flow stress becomes
constant after attain the maximum, since the equilibrium between generation and annihilation of dislocation
is established continuously by dynamic recovery. However, some authors have suggested that the dynamic
recrystallization can takes place in low carbon steels [1] and interstitial free steels (IF) [3]. In this work, three
grades of carbon steels and one interstitial free steel, whose chemical compositions are showed in table 1,
were subjected to torsion tests at 700o C and strained to 2.3 at strain rate of 1 s . After straining the
-1
specimens were air-cooled, Figure 1a. Although all materials were processed with the same parameters,
different microstructural features such as morphology and final grain size were identified due to the different
chemical composition of each alloy.
When samples of steels are submitted to large strains at intermedia ry temperatures by hot torsion test,
high grain size refinement has been attained at the surface of the specimens. Nevertheless, the smallest
microstructural change among all steels studied was observed on the external surface of strained samples
(Figures 1c; d; e; f), which represent the largest deformed region. Because of extensive deformation in this
region, dynamic recrystallization of ferrite occurred, resulting in a ferrite grain size of 2 µm. Also,-4
secondary recrystallization was observed in the la rgest deformation region in all samples. On the other hand,
at the smallest deformed region (Figures 1c’; d’; e’; f’), localized in the center of samples, significant
differences among the materials’ microstructure were observed: coarse ferrite grains for IF steel; composed
structure of ferrite and bainite surrounded by proeutectoid ferrite, for steels with carbon contents of 0.076
and 0.162%C; and martensite together with ferrite and bainite were observed inside of necklace formed by
proeutectoid ferrite in the alloy with 0.31%C.
In this work, it was observed that ferrite grain size became smaller after deformation on warm conditions
as the carbon content decreased and the material’s strength presented one opposite behavior, as showed in
the true stress-strain curves (Figure 1b). Nevertheless, the alloy elements present in the steels with higher
carbon (M2 and M1 steels) also act on grain refinement; see Figures 1 “e” and “f”. In this way, the grain
refinement of the M1 steel is more expressive than in M2 steel, even though the later contains less carbon
than the former.
The shapes of the flow curves of IF, M3 and M2 steels are a strong evidence that the ferrite softening by
intense dynamic recovery. By other hand, the presence of small grains (Figures 1c; d; e) after strain to 2.3
leads to conclusion that new grains were formed during the straining. Hence, the dynamic recrystallization in
these materials occurred; with the probable formation of new grains by subgrain growth and with the gradual
increase on the misorientation.
[1] HICKSON, M.R., GIBBS, R.K., HODGSON, D. ISIJ International, v.39, n.11, p.1176-1180, 1999.
[2] HURLEY, P.J. Thesis of the Doctorate of Philosophy. Monash University: Australia, Nov.1999, p.327.
[3] NAJAFI-ZADEH, A., JONAS, J.J., YUE, S. Metall. Transaction A, v.23A, p.2607-2617, Sep.1992.

2. Table 1 - Chemical composition (mass%), in balance with Fe
C Mn Si Al S P Ti Nb V Mo Cr B N Ni Cu
M1 0,31 1,51 - 0,024 0,032 0,65 0,026 0,0018 0,099 - - - - - -
M2 0,162 1,343 0,459 0,038 0,009 0,019 - - 0,030 - 0,011 0,0002 - 0,230 0,012
M3 0,076 1,36 0,49 0,011 0,0045 <0,005 <0,005 0,04 <0,01 0,01 0,07 - 0,004 0,19 0,03
IF 0,003 0,132 0,011 0,003 0,007 0,001 0,065 - - - - - 0,006 - -
(c’) IF (d’) M3
(a)
(c) (d)
300
250
200
Stress [MPa]
150
100
M1
M2
50 M3
IF
0
0,0 0,5 1,0 1,5 2,0 2,5
Strain
(b)
(e) (f)
(e’) M2 (f’) M1
Figure 1- (a) Schematic depiction of the thermomechanical processing route; (b) True stress-strain curve of the steels; (c) to (f)
Reflected light micrographs of the surface and centre regions of specimens of deformed steels.